Introduce a filter to allow BP directory pages status to be different than `publish`

[imath]

I believe to make the BP Rewrites plugin more attractive so that it get more users to test using BuddyPress with the WP Rewrite API, we should test some other features like the visibility one we discussed about here: #8734 (restricting BP pages to members). That's why I've been working on implementing it from this ​PR, I recently merged.

I noticed doing so it wasn't easy to use a different status than publish when querying for directory pages. I believe we should make it easier instead of filtering the query hook like I did ​there.

That's why I plan on introducing a filter to allow the use of other registered post stati.

Make it easier to use a different post status for BP Directory pages.

Trac ticket: ​https://buddypress.trac.wordpress.org/ticket/8772

[imath]

In 13377:

Introduce a function to get allowed post stati for BP directory pages

The bp_core_get_directory_pages_stati() function has been introduced to be able to get BP Directory pages having various registered post stati. So far only the publish post status was allowed. Filtering 'bp_core_get_directory_pages_stati', it's now possible to include other registered post stati. This new possibility gives a way to use WorPress Post Status to set custom & more granular user visibilities for components having a directory page.

Closes ​https://github.com/buddypress/buddypress/pull/40
Fixes #8772